Age- and sex-related dietary specialization facilitate seasonal resource partitioning in a migratory shorebird

Heavy metals in wetlands of southwestern India: from sediments through invertebrates to migratory shorebirds

Heavy metal pollution in Indian wetlands is rising due to industrial, agricultural and urban development activities. Shorebirds occupy upper trophic levels and are therefore especially vulnerable to heavy metal pollution. We evaluated the concentration of heavy metals (zinc, copper, cobalt, chromium, lead and cadmium) in 22 common species of migrant shorebirds (220 shorebird dropping samples) with diverse foraging behaviors, in their different prey (55 prey samples) and in the sediments (90 sediment samples) in different habitat types (mudflats, mangroves and sand beaches) between 2019 and 2021. Further, we analyzed a total of 10 biofilm samples from mudflats and mangroves. We detected relatively low concentrations of heavy metals in the sediments (Zn concentration range: 9.11-40.91 mg/kg; Cu: 5.74-21.38 mg/kg; Co: 2.00-4.04 mg/kg; Cr: 4.05-41.03 mg/kg; Pb: 1.02-7.19 mg/kg; Cd: 0.56-4.35 mg/kg). However, we measured relatively high concentrations of heavy metals in invertebrate prey species (Zn concentration range: 84.72-224.74 mg/kg; Cu: 26.63-170.36 mg/kg; Co: 13.98-14.42 mg/kg; Cr: 14.78-98.16 mg/kg; Pb: 18.95-157.29 mg/kg; Cd: 9.33-60.56 mg/kg). In addition, we found high concentrations of heavy metals in shorebird droppings (Zn concentration range: 41.33-58.8 mg/kg; Cu: 31.42-52.11 mg/kg; Co: 36.34-55.68 mg/kg; Cr: 52.3-68.21 mg/kg; Pb: 25.94-43.13 mg/kg; Cd: 5.53-16.4 mg/kg). It is evident that concentration of heavy metals increased successively moving from sediment to prey to shorebird species, likely through trophic transfer. The biofilm samples contained very high concentrations of Cr, Pb and Cd (22.64, 28.09 and 18.46 mg/kg respectively) which could be harmful to biofilm grazing shorebirds. Since bioaccumulation of heavy metals entail risks in living species, we suggest that increasing concentrations may detrimentally affect physiological processes in invertebrates and shorebirds. There is an urgent need to identify the sources of pollution and to reduce the discharge of heavy metals and other pollutants into coastal and inland wetlands.

Biofilm and invertebrate consumption by western sandpipers (Calidris mauri) and dunlin (Calidris alpina) during spring migratory stopover: insights from tissue and breath CO2 isotopic (δ 13C, δ 15N) analyses

Shorebirds use key migratory stopover habitats in spring and fall where body proteins are replenished and lipids stored as fuel for the remaining journey. The Fraser River estuary, British Columbia, Canada, is a critical spring stopover site for hundreds of thousands of migrating western sandpiper, Calidris mauri, and dunlin, Calidris alpina. Intertidal biofilm in spring is an important nutritional source for western sandpiper, with previous isotopic research predicting 45-59% of total diet and 50% of total energy needs. However, these studies relied on isotopic mixing models that did not consider metabolic routing of key dietary macromolecules. Complexity arises due to the mixed macromolecular composition of biofilm that is difficult to characterize isotopically. We expanded on these earlier findings by considering a protein pathway from diet to the body protein pool represented by liver tissue, using a Bayesian mixing model based on δ ¹³C and δ ¹⁵N. We used δ ¹³C measurements of adipose tissue and breath CO₂ to provide an estimate of the carbohydrate and protein δ ¹³C values of microphytobenthos and used these derived values to better inform the isotopic mixing models. Our results reinforce earlier estimates of the importance of biofilm to staging shorebirds in predicting that assimilated nutrients from biofilm contribute ~35% of the protein budgets for staging western sandpipers (n = 13) and dunlin (n = 11) and at least 41% of the energy budget of western sandpiper (n = 69). Dunlin's ingestion of biofilm appeared higher than anticipated given their expected reliance on invertebrate prey compared to western sandpiper, a biofilm specialist. Isotopic analyses of bulk tissues that consider metabolic routing and that make use of breath CO₂ and adipose lipid assays can provide new insights into avian physiology. We advocate further isotopic research to better understand biofilm use by migratory shorebirds in general and as a critical requirement for more effective conservation.